Enhanced linearity mixer

ABSTRACT

A system for enhanced linearity mixing includes an input-source signal coupler; a local oscillator (LO) signal coupler; a primary mixer that combines, via heterodyning, the primary-mixer-input signal and the primary-mixer-LO signal to generate a primary-mixer-output signal; a distortion-source mixer that combines, via heterodyning, the distortion-mixer-input signal and the distortion-mixer-LO signal to generate a distortion-mixer-output signal; and an output signal coupler that combines the primary-mixer-output signal and the distortion-mixer-output signal to generate an output signal with reduced non-linearity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 62/477,346, filed on 27 Mar. 2017 and of U.S. ProvisionalApplication Ser. No. 62/598,739, filed on 14 Dec. 2017, all of which areincorporated in their entireties by this reference.

TECHNICAL FIELD

This invention relates generally to the analog circuit field, and morespecifically to new and useful systems and methods for enhancedlinearity frequency mixing.

BACKGROUND

Traditional wireless communication systems are half-duplex; that is,they are not capable of transmitting and receiving signalssimultaneously on a single wireless communications channel. Recent workin the wireless communications field has led to advancements indeveloping full-duplex wireless communications systems; these systems,if implemented successfully, could provide enormous benefit to thewireless communications field. For example, the use of full-duplexcommunications by cellular networks could cut spectrum needs in half.One major roadblock to successful implementation of full-duplexcommunications is the problem of self-interference.

Many solutions to address self-interference rely on mixing circuits(e.g., as part of an analog self-interference canceller), but thesesolutions may suffer in performance due to constraints inherent intraditional frequency mixers. Thus, there is a need in the wirelesscommunications field to create new and useful systems and methods forenhanced linearity frequency mixing. This invention provides such newand useful systems and methods.

Of course, such systems and methods for enhanced linearity frequencymixing may find use in a wide variety of applications in analogcircuits.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram view of a system of an invention embodiment;

FIG. 2 is an example graph view of an output signal of a mixer;

FIG. 3 is an example view of signal combination of a system of aninvention embodiment;

FIG. 4 is an example view of signal combination of a system of aninvention embodiment;

FIG. 5 is an example view of signal combination of a system of aninvention embodiment;

FIG. 6 is an example view of signal combination of a system of aninvention embodiment; and

FIG. 7 is a diagram view of a system of an invention embodiment.

DESCRIPTION OF THE INVENTION EMBODIMENTS

The following description of the invention embodiments of the inventionis not intended to limit the invention to these invention embodiments,but rather to enable any person skilled in the art to make and use thisinvention.

1. Full-Duplex Wireless Communication Systems

Wireless communications systems have revolutionized the way the worldcommunicates, and the rapid growth of communication using such systemshas provided increased economic and educational opportunity across allregions and industries. Unfortunately, the wireless spectrum requiredfor communication is a finite resource, and the rapid growth in wirelesscommunications has also made the availability of this resource everscarcer. As a result, spectral efficiency has become increasinglyimportant to wireless communications systems.

One promising solution for increasing spectral efficiency is found infull-duplex wireless communications systems; that is, wirelesscommunications systems that are able to transmit and receive wirelesssignals at the same time on the same wireless channel. This technologyallows for a doubling of spectral efficiency compared to standardhalf-duplex wireless communications systems.

While full-duplex wireless communications systems have substantial valueto the wireless communications field, such systems have been known toface challenges due to self-interference; because reception andtransmission occur at the same time on the same channel, the receivedsignal at a full-duplex transceiver may include undesired signalcomponents from the signal being transmitted from that transceiver. As aresult, full-duplex wireless communications systems often include analogand/or digital self-interference cancellation circuits to reduceself-interference.

Full-duplex transceivers preferably sample transmission output asbaseband digital signals, intermediate frequency (IF) analog signals, oras radio-frequency (RF) analog signals, but full-duplex transceivers mayadditionally or alternatively sample transmission output in any suitablemanner (e.g., as IF digital signals). This sampled transmission outputmay be used by full-duplex transceivers to remove interference fromreceived wireless communications data (e.g., as RF/IF analog signals orbaseband digital signals). In many full-duplex transceivers, an analogself-interference cancellation system is paired with a digitalself-interference cancellation system. The analog self-interferencecancellation system removes a first portion of self-interference bysumming delayed, phase shifted and scaled versions of the RF transmitsignal to create an RF self-interference cancellation signal, which isthen subtracted from the RF receive signal. Alternatively, the analogcancellation system may perform similar tasks at an intermediatefrequency. After the RF (or IF) receive signal has the RF/IFself-interference cancellation signal subtracted, it passes through ananalog-to-digital converter of the receiver (and becomes a digitalreceive signal). After this stage, a digital self-interferencecancellation signal (created by transforming a digital transmit signal)is then subtracted from the digital receive signal.

The systems and methods described herein may increase performance offull-duplex transceivers (and other applicable systems) by enabling highlinearity frequency mixing without prohibitive increases in circuitcomplexity and/or cost. Other applicable systems include active sensingsystems (e.g., RADAR), wired communications systems, wirelesscommunications systems, channel emulators, reflectometers, PIM analyzersand/or any other systems featuring analog electronics, includingcommunication systems where transmit and receive bands are close infrequency, but not overlapping.

2. System for Enhanced Linearity Mixing

A system 100 for enhanced linearity mixing includes a primary mixer 110,a distortion-source mixer 120, and signal couplers 150, as shown inFIG. 1. The system 100 may additionally include a phase shifter 130, ascaler 140, and/or harmonic shorting circuits 160.

The system 100 functions to improve the linearity of frequency mixers(or more generally, circuits and line-ups including frequency mixers).High linearity circuits are important for a large variety of analogelectronic systems, particularly in communications systems.Traditionally, analog circuit designers can improve linearity bysourcing higher linearity components (which can incur significant cost),reducing power levels (which may have negative consequences forsignal-to-noise levels or otherwise), or by substantially increasingcircuit complexity and power consumption.

Operating on a general principle similar to the self-interferencecancellation techniques discussed in Section 1, the system 100 utilizescomponents (e.g., the distortion-source mixer 120) to model and subtractdistortion present in the output of the primary mixer 110 (or a moregeneral circuit including the primary mixer 110), thus creating a morelinear output of the system 100 than that of the primary mixer 110alone.

The primary mixer 110 functions to convert an input signal from a firstfrequency to a second frequency; e.g., from radio frequency (RF) tointermediate frequency (IF) or baseband, or from baseband to RF or IF,or from IF to baseband or RF.

The primary mixer 110 is preferably an active mixer, but mayadditionally or alternatively be a passive mixer. The primary mixer 110may comprise discrete components, analog integrated circuits (ICs),digital ICs, and/or any other suitable components. The primary mixer 110preferably functions to combine two or more electrical input signalsinto one or more composite outputs, where each output includes somecharacteristics of at least two input signals.

The primary mixer 110 preferably takes in an input signal as well as afrequency shift signal, preferably provided by a local oscillator (LO).The local oscillator is preferably a PLL (Phase Locked Loop) steereddigital crystal variable-frequency oscillator (VFO) but may additionallyor alternatively be an analog VFO or any other suitable type ofoscillator. The local oscillator preferably has a tunable oscillationfrequency but may additionally or alternatively have a staticoscillation frequency.

Given an input signal centered at frequency f1 and frequency shiftsignal at frequency f2, the primary mixer 110 may produce output signals(each a product of the input signal and the frequency shift signal) ateach of the following frequencies: f=nf₁+mf₂, where n and m areintegers. Take, for example, that f1 is 900 MHz, f2 is 750 MHz, and thedesired output frequency is 150 MHz. In this example, the problematicoutputs are those around 150 MHz, other from the primary output that isat f₁-f₂. In this example, the outputs other than the primary outputthat are near the desired frequency are at {{n, m}}={{−4,5}, {6,−7}}(which are, for most mixers, almost non-existent).

Unfortunately, the situation is more complicated when the primary mixerno encounters multiple closely spaced signals simultaneously (as iscommon in communications). Now assume two input signals at f1 and f2,and frequency shift at f3; now products can be produced at allf=nf₁+mf₂+of₃. Assuming now that f1 is 900.00 MHz, f2 is 900.050 MHz, f3is 750 MHz, and the desired output frequencies are 150.000 and 150.050MHz. Now, there are troubling outputs: {{n, m, o}}={{2,−1,−1},{−1,2,−1}} (third order terms), {{n, m, o}}={{3,−2,−1}, {−2,3,−1}}(fifth order terms), and {{n, m, o}}={{4,−3,−1}, {−3,4,−1}} (seventhorder terms). These outputs are as shown in FIG. 2.

The distortion-source mixer 120 functions to model the distortion of theprimary mixer 110 (e.g., as shown in FIG. 2). This output of thedistortion-source mixer 120 may then be subtracted from that of theprimary mixer 110, reducing the distortion present in the output of theprimary mixer 110.

The distortion present in the output of the primary mixer 110 is reducedbecause the signal power ratio of first order components to higher ordercomponents (i.e., components of order >1, also referred to as non-linearcomponents) in the distortion mixer output is preferably higher than inthe primary mixer output, so subtracting the distortion mixer outputfrom the primary mixer output reduces higher order components more thanit reduces first order components.

The distortion-source mixer 120 is preferably substantially similar tothe primary mixer 110, but the distortion-source mixer 120 may be amixer with different fundamental characteristics than the primary mixer110 (alternatively, they may be the same).

In a first configuration, the primary mixer 110 and distortion-sourcemixer 120 have substantially identical configuration and characteristics(e.g., input-referred third-order intercept point (IIP3), conversiongain, noise floor, frequency response) and substantially identical inputsignals. In this embodiment, the output of the distortion-source mixer120 may be attenuated relative to the primary mixer 110 (by the scaler140) and inverted (by the phase shifter 130) and then combined with theoutput of the primary mixer 110. However, in this invention embodiment,any reduction in distortion in the primary mixer 110 is accompanied byan equal reduction in the desired signal as well, as shown in FIG. 3(e.g., the desired signal and distortion are both reduced by 12 dB).This configuration is not desirable.

In a second configuration, the primary mixer 110 and distortion-sourcemixer 120 have substantially identical characteristics (e.g., IIP3,conversion gain, noise floor, frequency response), but different inputsignals. In this configuration, the input signal to thedistortion-source mixer 120 has a higher power than that of the primarymixer 110 (by some combination of splitting, attenuation, and/or gain).Because the third order intermodulation products roughly grow with inputpower to the third order (and so on for fifth and seventh orderproducts), in this configuration, the increased input power means thatthe signal produced by the distortion-source mixer 120 is morenon-linear than that of the primary mixer 110. The output of thedistortion-source mixer 120 may then be attenuated (or the primary mixer110 signal may be amplified) before subtraction. This may be a desirableconfiguration of the system 100. An example is as shown in FIG. 4. Notethat this technique may possibly be limited by the higher orderintermodulation products; that is, if the signal (gain) is increasedenough on the distortion-source mixer 120 input, it may result in theaddition of noise, as shown in FIG. 5.

Note that due to manufacturing variance, substantially similarcharacteristics may mean that the mixers share identical characteristicspecifications (e.g., each characteristic parameter has an identicalcenter value and identical error ranges) but are not actually identical(e.g., both mixers may have an insertion loss of 3 dB plus or minus 0.5dB, meaning that one mixer could have an insertion loss of 3.1 dB whileanother has an insertion loss of 2.7 dB).

A variation of the second configuration is using identical input signalsbut different LO signal levels. When a lower LO level is used for thedistortion-source mixer its non-linearity will increase and so will theintermodulation products. The result is similar to the plots shown inFIG. 4 (or FIG. 5).

Both methods described for the second configuration may be combined tooptimize linearity, insertion loss, circuit complexity and noise figure.

In a third configuration, the primary mixer 110 and distortion-sourcemixer 120 have non-identical configuration and/or characteristics (e.g.,IIP3, conversion gain, noise floor, frequency response, operating mode),but substantially identical input signals. For example, the primarymixer 110 and distortion-source mixer 120 may have similar conversiongains and noise floors, but a different IIP3. In this example, thedistortion-source mixer 120 preferably exhibits non-linearity similar inform but of a greater magnitude than of the primary mixer 110, allowingfor similar effects to the second configuration, but without necessarilysuffering the same limitations of the second configuration (e.g.,requiring both higher power and a mixer to handle it). In fact, in somemixers, a “low-power” mode enables the mixer to operate at a loweroperating power, but with lower IIP3; the system 100 may utilize aprimary mixer 110 in “normal mode” and a distortion-source mixer 120 in“low-power” mode in such a scenario. This may be a desirableconfiguration of the system 100. An example is as shown in FIG. 6.

The system 100 may additionally or alternatively use both mixers 110/120with non-identical characteristics, non-identical input signals andnon-identical LO signals. Mixers 110/120 may be configured in any mannerand are not limited to the examples given.

Note that as shown in FIG. 1, the primary mixer 110 anddistortion-source mixer 120 share a local oscillator source;additionally or alternatively, the primary mixer 110 anddistortion-source mixer 120 may utilize different local oscillatorsignals.

The phase shifter 130 preferably functions to shift the phase of one ofthe primary mixer 110 and distortion source mixer 120 such that theoutput of the distortion source mixer 120 is 180 degrees out of phasewith the primary mixer 110 before addition of the signals.Alternatively, the phase shifter 130 may be used for any phase shiftingpurpose.

The phase shifter 130 may include an impedance matching network at itsinput and output that compensates for variations in the phase shifter130 input and output impedance (and/or phase shift amount) due tochanges in signal component frequency or simply transforms the impedanceto and from a suitable impedance level for the core of the phase shifterto a standardized impedance level (50 ohms). Alternatively, the phaseshifter 130 may not include impedance matching networks. The impedancematching networks are preferably tunable (e.g., continuously ordiscretely variable) but may additionally or alternatively be static(i.e., the impedance transformation achieved by using the network is notvariable).

The phase shifter 130 is preferably separated into a set of phaseshifting stages. These phase shifting stages preferably may be switched‘on’ (e.g., in signal path) or ‘off’ (e.g., bypassed, out of signalpath), depending on control signals. The resulting phase shift isdetermined by which stages are on and which stages are off; for example,a phase shifter 130 with a 90-degree phase shifting stage and a10-degree phase shifting stage ‘on’ might cause a shift of 100 degreesin signal phase.

Each phase shifting stage preferably causes a set amount (i.e.,non-variable amount) of phase shift. Alternatively, phase shiftingstages may include tunable phase-shift elements. For example, a phaseshifting stage may include a varactor; by changing a control voltage ofthe varactor, the varactor's capacitance (and thus the amount of phaseshift experienced by a signal passing through the stage) may be varied.

The phase shifters 130 are preferably controlled by a tuning circuit,but may additionally or alternatively be controlled in any suitablemanner.

Note that phase shifters 130 may be located at any point in the system100; e.g., between the LO and the primary mixer 110 input on the LOsignal; between the LO and the distortion-source mixer 120 input on theLO signal; between the system input and the primary mixer 110 input onthe input signal; between the system input and the distortion-sourcemixer 120 input on the input signal; between the primary mixer 110output and the system output; and/or between the distortion-source mixer120 output and the system output.

The scaler 140 functions to scale transmit signal components;specifically, the scalers 140 effectively multiply the transmit signalcomponents by a scale factor. For example, an attenuation of 34% mightbe represented as a scale factor of 0.66; a gain of 20% might berepresented as a scale factor of 1.20; and an attenuation of 10% and aphase inversion might be represented as a scale factor of −0.90. Scalefactors may be complex; for example, a scale factor of

might be represented as a phase shift of ninety degrees.

The scalers 140 may include attenuators, amplifiers, phase inverters,and/or any other suitable components for scaling analog signalcomponents. Attenuators may be resistive attenuators (T pad, Pi pad),amplifiers with less than unity gain, or any other suitable type ofattenuator. Amplifiers may be transistor amplifiers, vacuum tubeamplifiers, op-amps, or any other suitable type of amplifier. Phaseinverters may be any phase inversion devices, including NPN/PNP phaseinversion circuits, transformers and/or inverting amplifiers.

The scalers 140 preferably are capable of attenuation, gain, and phaseinversion, but may alternatively be capable only of a subset of saidcapabilities. Each scaler 140 preferably includes all three capabilitiesin a single device (e.g., an amplifier with tunable gain and twooutputs, one inverted and one non-inverted) but may additionally oralternatively separate capabilities into different sections (e.g., anamplifier with tunable gain but no inversion capability, along with aseparate phase inversion circuit, an attenuator). The scalers 140 arepreferably controlled by a tuning circuit, but may additionally oralternatively be controlled in any suitable manner.

Similarly to phase shifters 130, scalers 140 may be located at any pointin the circuit. For example, as shown in FIG. 1, an attenuating scaler140 may be used to attenuate the output of the distortion-source mixer120. Additionally or alternatively, an amplifying scaler 140 could beused to amplify the output of the primary mixer 110.

Note that in some cases functionality (e.g., in phase inversion) may beaccomplished by either or both of the phase shifter 130 and the scaler140.

Signal couplers 150 function to allow analog signals to be split and/orcombined. Signal couplers 150 may couple and/or split signals usingvarying amounts of power; for example, a signal coupler 150 intended tosample a signal may have an input port, an output port, and a sampleport, and the coupler 150 may route the majority of power from the inputport to the output port with a small amount coming from the sample port(e.g., a 99.9%/0.1% power split between the output and sample port, orany other suitable split).

The signal coupler 150 is preferably a short section directionaltransmission line coupler, but may additionally or alternatively be anypower divider, power combiner, directional coupler, or other type ofsignal splitter. The signal coupler 150 is preferably a passive coupler,but may additionally or alternatively be an active coupler (forinstance, including gain blocks and power amplifiers). For example, thesignal coupler 150 may comprise a coupled transmission line coupler, abranch-line coupler, a Lange coupler, a Wilkinson power divider, ahybrid coupler, a hybrid ring coupler, a multiple output divider, awaveguide directional coupler, a waveguide power coupler, a hybridtransformer coupler, a cross-connected transformer coupler, a resistivetee, and/or a resistive bridge hybrid coupler.

For example, a signal coupler 150 may split an input-source signal (theinput to the system 100) into two input signals, one of which will serveas input to the primary mixer 110 (primary-mixer-input signal) and oneof which will serve as input to the distortion-source mixer 120(distortion-mixer-input signal). Likewise, another signal coupler 150may split a local oscillator signal into two signals, one of which willserve as the LO signal for the primary mixer 110 (primary-mixer-LOsignal) and one of which will serve as the LO signal to thedistortion-source mixer 120 (distortion-mixer-LO signal).

Harmonic shorting circuits 160 function to reduce the contribution ofharmonics to the output of the system 100 (and thus function to increaselinearity of the output). Harmonic shorting circuits are preferablyseries LC resonators tuned to resonance at a specific harmonic frequency(e.g., 3f, 5f), but may additionally or alternatively be any circuitcapable of shorting a signal path of the system 100 at a specificdesired frequency. Similar to phase shifters 130 and scalers 140,harmonic shorting circuits 160 may be placed at any point in the system100. For example, a third harmonic short may be placed in the outputpath of the distortion-source mixer 120 (which reduces the presence offifth-order intermodulation (IM5) and/or seventh-order intermodulation(IM7) products in the output of the distortion source mixer 120 andprevents a growth of these components after subtraction as shown in FIG.5). A harmonic shorting circuit 160 may additionally or alternatively beplaced at the input or output of the primary mixer 110, at the output ofthe distortion-source mixer 120, or at any other location. Note thatmultiple harmonic shorting circuits 160 may be placed in parallel (orotherwise may be located in the system 100) to reduce the presence ofharmonics at multiple frequencies.

The methods of the preferred embodiment and variations thereof can beembodied and/or implemented at least in part as a machine configured toreceive a computer-readable medium storing computer-readableinstructions. The instructions are preferably executed bycomputer-executable components preferably integrated with a system forenhanced linearity mixing. The computer-readable medium can be stored onany suitable computer-readable media such as RAMs, ROMs, flash memory,EEPROMs, optical devices (CD or DVD), hard drives, floppy drives, or anysuitable device. The computer-executable component is preferably ageneral or application specific processor, but any suitable dedicatedhardware or hardware/firmware combination device can alternatively oradditionally execute the instructions.

As a person skilled in the art will recognize from the previous detaileddescription and from the figures and claims, modifications and changescan be made to the preferred embodiments of the invention withoutdeparting from the scope of this invention defined in the followingclaims.

We claim:
 1. A system for enhanced linearity mixing comprising: aninput-source signal coupler that splits a first input signal into aprimary-mixer-input signal and a distortion-mixer-input signal; a localoscillator (LO) signal coupler that splits a first LO signal into aprimary-mixer-LO signal and a distortion-mixer-LO signal; a primarymixer that combines, via heterodyning, the primary-mixer-input signaland the primary-mixer-LO signal to generate a primary-mixer-outputsignal that comprises a first-order primary-mixer-output signalcomponent and a higher-order primary-mixer-output signal component;wherein the primary-mixer-output signal is characterized by a firstsignal power ratio of the higher-order primary-mixer-output signalcomponent to the first-order primary-mixer-output signal component; adistortion-source mixer that combines, via heterodyning, thedistortion-mixer-input signal and the distortion-mixer-LO signal togenerate a distortion-mixer-output signal that comprises a first-orderdistortion-mixer-output signal component and a higher-orderdistortion-mixer-output signal component; wherein thedistortion-mixer-output signal is characterized by a second signal powerratio of the higher-order distortion-mixer-output signal component tothe first-order distortion-mixer-output signal component; wherein thesecond signal power ratio is greater than the first signal power ratio;and an output signal coupler that combines the primary-mixer-outputsignal and the distortion-mixer-output signal to generate an outputsignal that comprises a first-order output signal component and ahigher-order output signal component; wherein the output signal ischaracterized by a third signal power ratio of the higher-order outputsignal component to the first-order output signal component; wherein thethird signal power ratio is lesser than the first signal power ratio andlesser than the second signal power ratio.
 2. The system of claim 1,further comprising a phase shifter; wherein the phase shifter invertsone of the primary-mixer-input signal, thedistortion-mixer-input-signal, the primary-mixer-LO signal, and thedistortion-mixer-LO signal; wherein inversion results in theprimary-mixer-output signal being inverted relative to thedistortion-mixer-output signal at the output signal coupler.
 3. Thesystem of claim 2, wherein the phase shifter inverts theprimary-mixer-input signal.
 4. The system of claim 2, wherein the phaseshifter inverts the primary-mixer-LO signal.
 5. The system of claim 2,wherein the phase shifter inverts the distortion-mixer-input signal. 6.The system of claim 2, wherein the phase shifter inverts thedistortion-mixer-LO signal.
 7. The system of claim 1, wherein thehigher-order signal components are third-order intermodulation products.8. The system of claim 1, wherein the higher-order signal components arefifth-order intermodulation products.
 9. The system of claim 1, whereinthe distortion-source mixer and primary mixer have identicalcharacteristic specifications.
 10. The system of claim 9, wherein thedistortion-mixer-input signal has higher signal power than theprimary-mixer-input signal, resulting in the distortion-mixer-outputsignal having higher non-linearity than the primary-mixer-output signal.11. The system of claim 10, further comprising a first scaler thateither attenuates the primary-mixer-input signal or amplifies thedistortion-mixer-input signal.
 12. The system of claim 11, furthercomprising a second scaler that either attenuates thedistortion-mixer-output signal or amplifies the primary-mixer-outputsignal, such that the first-order primary-mixer-output signal componentis higher power than the first-order distortion-mixer-output signalcomponent.
 13. The system of claim 9, wherein the distortion-mixer-LOsignal has lower signal power than the primary-mixer-LO signal,resulting in the distortion-mixer-output signal having highernon-linearity than the primary-mixer-output signal.
 14. The system ofclaim 10, further comprising a first scaler that either amplifies theprimary-mixer-LO signal or attenuates the distortion-mixer-LO signal.15. The system of claim 11, further comprising a second scaler thateither attenuates the distortion-mixer-output signal or amplifies theprimary-mixer-output signal, such that the first-orderprimary-mixer-output signal component is higher power than thefirst-order distortion-mixer-output signal component.
 16. The system ofclaim 9, wherein the distortion-source mixer is operating in a differentoperating mode than the primary mixer, resulting in thedistortion-mixer-output signal having higher non-linearity than theprimary-mixer-output signal.
 17. The system of claim 16, wherein thedistortion-source mixer is operating in a low-power mode.
 18. The systemof claim 8, wherein the distortion-source mixer and primary mixer havenon-identical characteristic specifications.
 19. The system of claim 18,wherein the distortion-source mixer and primary mixer have differentinput-referred third-order intercept point (IIP3) values.
 20. The systemof claim 1, further comprising a harmonic shorting circuit coupled to anoutput of the distortion-source mixer that reduces at least one offifth-order intermodulation (IM5) products and seventh-orderintermodulation (IM7) products in the distortion-mixer-output signal.21. The system of claim 20, wherein the harmonic shorting circuitcomprises at least one LC resonator tuned to a fifth-order harmonicfrequency.
 22. The system of claim 21, wherein the harmonic shortingcircuit comprises at least one LC resonator tuned to a seventh-orderharmonic frequency.